In the manufacture of tires the unvulcanized tire is placed into a mold, the mold has surfaces for forming the tread and sidewalls of the tire.
There are conventionally two basic types of tire molds; the first type is a two piece mold the second type is a segmented mold.
The two-piece mold is considered the least expensive type, generally. The mold has two halves, an upper mold half and a lower mold half, generally these halves meet at the tread center or equatorial plane of the tire. The mold halves typically open similar to a clamshell. The molds typically work best for tires having minimal blading, particularly in the tread shoulders.
When superior quality tires or more complex treads having shoulder blading and intricate grooves are needed then the use of segmented molds is preferred. Segmented molds generally have radially expanding and contracting segments for forming the tread. The segments form an annular ring when the mold is closed. When the mold is opened the segments are moved radially outwardly to release the tread from the segments. As can be appreciated such a mold permits blades and circumferential and laterally inclined groves forming ribs to be used in the mold segments. The segments are radially expanded forcing the segments from the freshly molded tread pattern without cutting the tread elements. Unlike the two piece mold, the tread elements do not need to deform out of the way of these ribs and blades as the mold opens.
These features or benefits of a segmented type mold come with a somewhat higher cost of the mold.
In most tire molding applications the segmented mold also has two annular sidewall forming rings or plates. These sidewall rings of the mold can open similar to the two piece mold. The primary difference is the radially outer extremes of the two annular rings are abuttingly in contact with the tread forming segments when the mold is closed. The primary benefit to this mold is the segments can be made with a minimum concavity limited to the tread itself. In most passenger and light truck tires the tread extends radially inwardly to a very short distance called the tread shoulder. Thereafter, the tire sidewalls having indicia and other decorative bands are formed extending from the tread shoulders to the tire beads.
It has become advantageous to form the tread extending much further down the tire sidewall. The Goodyear Tire & Rubber company has developed a tire called the Wrangler MT/R which has the shoulder tread elements protruding outwardly and extending down the sidewall to a location 75% of the section height of the tire above the nominal rim diameter. This on/off road light truck tire employs extended shoulder traction elements to enhance off-road traction.
In order to make this tire, the tread forming segments of the mold were extended greatly increasing the depth of the concavity of the segments. Correspondingly, the resultant sidewall plates or rings were reduced diametrically. The resultant tire had a very distinctive appearance, but at a fairly large increase in tooling cost.
The present invention has provided a new way to fabricate segmented molds with greatly reduce construction costs while at the same time providing extended axially outer ends of shoulder tread elements.
Besides providing a lower cost way to make such a tire, the present invention teaches a novel method of increasing the visual appearance of the tread shoulder.